1. Field of the Invention
This invention relates to the field of radar and, more particularly, to a radar system which employs a segmented chirp waveform implemented transmitter. Such a radar system is particularly suitable for use in conjunction with or as a digital image processor.
2. Description of the Prior Art
Imaging waveforms and, to a lesser extent, waveforms for moving target detection, require extremely high time-bandwidth products. Modern high resolution spotlight imaging radars use waveforms with time-bandwidth products as high as 5.times.10.sup.9 (cycles). Classically, the use of a transmitted coherent train of precision wideband chirp pulses evolved as a type of waveform which is compatible with optical image processors. More recently, digital image processors have become popular and are often required to process radar returns originating from a train of wideband chirp pulses. This wide bandwidth imposes very high sampling rates on the collection of the digital data.
It is well known that the range resolution of a transmitting radar waveform is inversely proportional to the waveform bandwidth and that the doppler resolution is inversely proportional to the waveform duration. Hence, the need for a large time-bandwidth product for the total waveform is apparent. This need is not suitable for many applications that use digital processing because high sampling rates are not consistent with the analog-to-digital converters, data links and digital signal processors, especially in applications where such equipment is designed to be time shared with other functions.
Wideband chirp pulse transmitters have other shortcomings. Doppler shifts really depend not on carrier frequency, but on actual instantaneous frequency. For high resolution radars using very wide bandwidth waveforms, this can be a non-negligible effect and must be accounted for in the compensation processing. However, a single return has many reflected signals superimposed thereon and these cannot be separately compensated. Thus, one compensates only the carrier doppler and this limits resolution.
Wideband chirp-pulse-implemented radar systems also have accuracy problems. Extreme linearity is required and, in very high resolution systems, elaborate compensation is needed. Consequently, high resolution, bistatic synthetic aperture radar has not been feasible.
One attempt at improving radar system performance in a stacked beam configuration which utilizes a chirp generator is described in U.S. Pat. No. 3,344,426 issued to L. Long. In this arrangement, each chirp pulse is divided into a plurality of sub-pulses, each having a different frequency and width. The sub-pulses are then used to provide illumination energy at each elevation angle and thereby permit a large number of stacked beams to be illuminated by a single transmitted pulse.
U.S. Pat. No. 4,037,159 issued to A. Martin describes a communication system wherein a transmitter is adapted to forward successive chirp signals which start at different randomly determined initial frequencies. All of the chirp signals have the same frequency-time slope. Due to the action of a predetermined program known at a receiver station, only certain of the available chirp pulses are selected for transmission. There is no mention of chirp pulse bandwidth consideration, frequency spacing or suitability for use in radar systems.
In U.S. Pat. No. 3,898,660 issued to A. Munster, a pulsed sensor system is described wherein target response chirp pulses are first offset in frequency linearly with range, delayed as a function of frequency and then finally rechirped into narrow band chirp pulses suitable for processing. This action is taken to effect time-bandwidth interchange in order to allow the use of a low speed A/D converter.
It will be realized by those skilled in this art, that conventional chirp pulse generator implemented radar systems will not overcome the difficulties noted heretofore. The same can be said of a radar system incorporating a chirp generator which would operate as described in the foregoing prior art patents.